D. I. Southern

X and Y chromosomes of Aedes aegypti (L.) distinguished by Giemsa C-banding

A Giemsa C-banding technique applied to the mosquito, Aedes aegypti, has revealed a distinctive banding pattern which is described as a reliable means of distinguishing between the morphologically similar X and Y chromosomes during all stages of mitosis and meiosis. The essential difference is that the Y chromosome, unlike the X and the autosomes, is not C-banded in the centromere region. An intercalary band is also present in one arm of all X chromosomes and some Y chromosomes. The distribution of these cytological markers throughout meiosis indicates that the sex locus occurs somewhere within a pericentric region, the minimum extent of which includes both the intercalary band and the centromere.

A cytogenetic analysis of meiotic drive in the mosquito, Aedes aegypti (L.)

Meiotic drive in Aedes aegypti (L.) is shown by a Giemsa C-banding technique to be associated with preferential isochromatid breakage of the X chromosome during male meiosis. These breaks remain open at least until anaphase-I and, since the range of cells affected is proportional to the sensitivity of the X chromosome to the Distorter gene, it is argued that they are directly related to the decreased number of spermatozoa found in distorting males. This reduction is considered to be attributable to the degeneration of more X- than Y-bearing spermatids but it is probable that some non-functional X-bearing spermatozoa are also produced. Chromosome breakage is almost completely confined to four sites, two adjacent to the centromere, one just proximal to the intercalary band and another about the centre of the unbanded arm. Although the first three of these lie within a region in which crossing-over does not take place, fragmentation occurs more frequently in a chiasmate arm than in one devoid of chromatid exchange.

Stable telocentric chromosomes produced following centric misdivision in Myrmeleotettix maculatus (Thunb.)

The standard complement of Myrmeleotettix maculatus includes a pair of L3 metacentric chromosomes but throughout the germ line of a structurally mutant male individual one of these had been replaced by two telocentrics. These chromosomes divided normally at mitosis and at meiosis paired completely with the arms of the remaining L3. There was no tendency for the two terminal centromeres to fuse. At first anaphase the two rods usually segregated from the metacentric which resulted in the formation of a high percentage of balanced gametes. On the basis of this evidence and the morphological appearance of the centric region it has been concluded that the derived telocentrics evolved following misdivision through the inner zone of the metacentric centromere. In view of their apparent stability it has been proposed that, contrary to certain beliefs, centric fission may have played an important role during karyotype evolution in the Acrididae.

Chromosome relationships and meiotic mechanisms of certain morsitans group tsetse flies and their hybrids

There are three pairs of euchromatic components, the L1 and L2 autosomes and the X chromosome pair respectively, which are found in both G. austeni and the three forms of G. morsitans. Each species/sub-species also includes in its complement a group of heterochromatic autosomes (S) which have various morphologies and differ in number both within and between the species/sub-species. Several lines of evidence are outlined which point to these being supernumerary B chromosomes. Male meiosis is normally achiasmate and only L1 and L2 autosomes pair completely. X-Y association is restricted to a small pairing segment the position of which on the X is constant for all the species/sub-species. It is located in one of two positions on the Y chromosome according to the species/sub-species. The S chromosomes behave as hereditary univalents at first anaphase while the sex bivalent can undergo “distance pairing” best exemplified in G. austeni and G. submorsitans. A Y structural mutant line gives some indication of the size of the pairing segment and demonstrates that survival and maleness is possible even when two-thirds of the chromosome is missing. Meiotic and polytene chromosome studies connected with hybridisation experiments designed to test the sterility factor as a potential means of tsetse control assist in establishing the evolutionary relationship of the subspecies.

Cytological mapping of the M and D loci in the mosquito, Aedes aegypti (L.)

The separate identities of the male-determining factor, M, and the sex-linked Distorter gene, D, are established in an Accra strain of Aedes aegypti. Their to each other and to Giemsa C-bands. Thus, M is invariably inherited with the centromere, whereas D lies towards the intercalary band. Approximately 1.2% recombination occurs between M and D but, in a chromosome known for its distal localization of chiasmata, it is argued that he two are not necessarily as closely linked cytologically as this might imply. Evidence on the genetic effects of recombination in the region of M and D is also considered.

Comparative analysis of the polytene chromosomes of Glossina austeni and Glossina morsitans morsitans.

Polytene chromosome maps of the tsetse fly Glossina austeni have been prepared and a description of all the principal chromosome markers is provided. A detailed comparison has been effected between these chromosomes and the polytene elements of G. m. morsitans the only other member of the same sub-genus so far mapped. Although there are many similarities in banding pattern a relatively large number of paracentric inversions and at least one pericentric inversion separate the two species. There are also groups of bands in both species which are not shared in common.

Polymorphism involving heterochromatic segments in Metrioptera brachyptera.

A complex pattern of polymorphism involving terminal heterochromatic segments on L3 and L4 chromosomes has been uncovered in eight populations of Metrioptera brachyptera. There are individuals in every population which carry reduced segments on one or both L4s. In six populations, enlarged heterochromatic segments have been encountered on the L3 chromosomes in some individuals. The L4 system is almost certainly stable although the frequency of L4 karyotypes does not conform to a Hardy-Weinberg distribution in all populations. Stability of the L3 polymorphism could not be ascertained. A reduction of L4 heterochromatin leads to a significant rise both in mean cell chiasma frequency and between cell variance. The effect on chiasma frequency is transchromosomal. The normal pattern of strict chiasma localisation tends to be disrupted in germ lines which include modified L4 chromosomes. There is a reduction in the number of proximal and distal chiasmata and an increase in the frequency of interstitial ones. It is proposed that the standard L4 heterochromatin may function in conserving heterozygosity and promoting uniformity between parent and offspring. Partial removal may lead to an effective increase in recombination and produce a greater diversity of genotypes for selection to act upon.

Polytene chromosomes of the tsetse fly Glossina morsitans morsitans.

Cytological maps have been prepared, and a detailed description is presented of all major markers in the polytene chromosomes of the tsetse fly Glossina morsitans morsitans, an important vector of African trypanosomiases. Only the five eu-chromosomes in males (L1 and L2 autosome pairs plus X chromosome) and the six eu-chromosomes in females of the species (L1, L2 and X chromosome pairs) form polytene elements. The group of heterochromatic S autosomes and the Y chromosome appear to be absent in polytene nuclei. The information contained in this paper may further the development of genetical control methods of tsetse flies, and should assist in phylogenetic analyses of the genus Glossina.

Persistent heterochromatic association in Metrioptera brachyptera. I. Variation in the frequency of multiple formation, chiasma production and chromosome morphology.

The non-homologous and achiasmate association of L3 and L4 chromosomes leading to the formation of persistent pseudo-multiples has been detected in each of the natural populations sampled. Populations exhibit different mean frequencies and within a given sample a considerable range may be found. The mean chiasma frequencies of populations vary significantly but there is no correlation between these and the level of multiple formation. It is proposed that the mechanism controlling chiasma production and that determining the frequency of multiples act independently of one another. Individuals have been discovered where either one or both members of the L3 or L4 pair have been modified structurally. An analysis of these entire germ line conditions indicates that not only is heterochromatin primarily concerned in the formation and maintenance of multiples, but also, that their inception is correlated with the distal position of these segments on L3 and L4 chromosomes.The non-homologous and achiasmate association of L3 and L4 chromosomes leading to the formation of persistent pseudo-multiples has been detected in each of the natural populations sampled. Populations exhibit different mean frequencies and within a given sample a considerable range may be found. The mean chiasma frequencies of populations vary significantly but there is no correlation between these and the level of multiple formation. It is proposed that the mechanism controlling chiasma production and that determining the frequency of multiples act independently of one another. Individuals have been discovered where either one or both members of the L3 or L4 pair have been modified structurally. An analysis of these entire germ line conditions indicates that not only is heterochromatin primarily concerned in the formation and maintenance of multiples, but also, that their inception is correlated with the distal position of these segments on L3 and L4 chromosomes.

Relative DNA content of normal and sex-ratio distorting spermatozoa of the mosquito,Aedes aegypti (L.)

Scanning microdensitometry inAedes aegypti (L.) has revealed an extended range of DNA levels among the spermatozoa of sex-ratio distorting males compared with their normal counterparts. Fewer than 1% of spermatozoa contain less than the 1C level of DNA but more than 20% include greater quantities. Since this variation is identified with morphologically abnormal spermatozoa, it is considered to be a direct consequence of preferential X-chromosome breakage during male meiosis and hence of meiotic drive.